We propose to immunize mice or rats with transformed cells, fuse their spleen cells with mouse myeloma cells and select hybridomas producing antibodies binding to the cell line used for immunization but not to normal cells. The following pairs of immunizing cells and control normal cells will be used: (1) human melanoma/autologous fibroblasts, (2) in vitro transformed human fibroblasts/parental fibroblast line, (3) in vitro transformed C3H/10T one-half fibroblasts (CL15, CL16)/ C3H/10T one-half fibroblasts (CL8), (4) UV-induced C3H fibrosarcomas/C3H/10T one-half fibroblasts. Putative transformation-associated antigens identified in this way will be studied further by binding assays with radioiodinated monoclonal antibody and by immunoprecipitation. Antigens of particular interest will be characterized structurally (peptide mapping, aminoacid sequence analysis), and their relationship to the transformed state will be determined.